Alcohol & Phenol
Hydroxyl-containing organic compounds: alcohols ($\text{R-OH}$) have the $-\text{OH}$ attached to an $\text{sp}^3$ hybridized carbon, while phenols have the $-\text{OH}$ attached directly to an aromatic ring.
[!important] Phenol is NOT an alcohol and NOT an aromatic alcohol.
SMILES Gallery
CO
Methanol
CCO
Ethanol
CC(C)O
Propan-2-ol (isopropyl alcohol)
CC(C)(C)O
tert-Butyl alcohol (2-methylpropan-2-ol)
c1ccccc1O
Phenol
c1ccccc1CO
Phenylmethanol (benzyl alcohol) — an aromatic alcohol, NOT a phenol
C1CCCCC1O
Cyclohexanol
O=[N+]([O-])c1ccccc1O
2-Nitrophenol
O=[N+]([O-])c1ccc(O)cc1
4-Nitrophenol
Nc1ccccc1O
2-Aminophenol
Nc1ccc(O)cc1
4-Aminophenol
Cc1ccc(O)cc1
4-Methylphenol (p-cresol)
Fc1ccc(O)cc1
4-Fluorophenol
Classification of Alcohols
Alcohols are classified by the carbon atom bearing the $-\text{OH}$ group (the alcoholic carbon), not by the number of R groups on oxygen.
| Type | Structure | Oxidation Product |
|---|---|---|
| Primary (1°) | $\text{R-CH}_2\text{-OH}$ | Aldehyde → Carboxylic acid |
| Secondary (2°) | $\text{R}_2\text{CH-OH}$ | Ketone |
| Tertiary (3°) | $\text{R}_3\text{C-OH}$ | Resistant to oxidation |
[!warning] Contrast with amines Amine classification depends on R groups attached to N, not carbon.
Nomenclature
IUPAC Names
- Determine longest chain containing $-\text{OH}$.
- Number from end closer to $-\text{OH}$.
- Replace -e of parent alkane with -ol.
- Denote $-\text{OH}$ position with number.
- Cyclic alcohols: numbering begins from carbon bearing $-\text{OH}$.
- Multiple $-\text{OH}$: diol, triol, tetraol.
Common Names
- Alcohol named as alkyl derivative of water.
- Examples: methyl alcohol, ethyl alcohol, isopropyl alcohol.
Isomerism
Structural Isomers
- Chain isomerism: different carbon skeleton
- Positional isomerism: different $-\text{OH}$ position
- Functional group isomerism: alcohol vs ether (e.g. ethanol vs dimethyl ether)
Optical Isomers
- Require stereocenter(s) (chiral carbon with 4 different groups).
- Enantiomers rotate plane-polarized light in equal but opposite directions.
- Example: butan-2-ol exists as $(+)$/$d$ and $(-)$/$l$ enantiomers.
Physical Properties
Boiling Point Discussion Framework
When comparing b.p. across compounds, analyze in this priority:
- Hydrogen bonding — strongest influence; requires H bonded to O/N/F
- Dipole-dipole attractions — net dipole moment $\mu$
- Exposed surface area — straight chain > branched
- Relative molar mass (RMM) — more atoms → more vdW forces
General order: $\text{H-bond} > \text{dipole-dipole} > \text{vdW}$
Key Trends
- Alcohols have much higher b.p. than hydrocarbons/ethers of similar RMM due to H-bonding.
- Ethanol (b.p. 78 °C) vs propane (b.p. −42 °C): ~120 °C difference.
- Ethanol vs dimethyl ether (b.p. −24 °C): H-bonding contributes >100 °C vs dipole-dipole alone.
- Primary > Secondary > Tertiary for same RMM: exposed surface area decreases with branching.
- Phenols > aliphatic analogues: electron delocalization strengthens intermolecular attractions.
Intramolecular vs Intermolecular H-Bonding
- 2-nitrophenol (intramolecular): b.p. 217 °C
- 3-nitrophenol (mixed): b.p. 230 °C
- 4-nitrophenol (intermolecular): b.p. 245 °C
Solubility
- 1–3 carbon alcohols: completely soluble in water.
- 4–10 carbon alcohols: oily liquids, decreasing solubility.
-
11 carbon alcohols: almost insoluble solids.
- Solubility decreases with RMM (longer hydrophobic tail).
- For same RMM: more $-\text{OH}$ groups → higher solubility.
- Phenol: partially soluble below 66 °C, completely soluble above (compact shape + strong H-bonds with water).
Acidity of Alcohols
Alcohols are weak acids. The acid-dissociation constant:
$$K_a = \frac{[\text{H}_3\text{O}^+][\text{RO}^-]}{[\text{ROH}]}$$
- Higher $K_a$ = stronger acid; higher $pK_a$ = weaker acid.
Factors Affecting Acidity
1. Inductive Effects
- EWG in R (e.g. halogens): stabilize alkoxide, increase polarity of O–H → stronger acid.
- EDG in R (alkyl groups): destabilize alkoxide, decrease polarity → weaker acid.
2. Solvation Effects
- Bulky R groups inhibit solvation of alkoxide and hinder proton abstraction by water.
- More R groups → weaker acidity (complementary to inductive effect).
pKa Values
| Compound | $pK_a$ | Notes |
|---|---|---|
| HCl | ~−7 | Strong acid (reference) |
| Acetic acid | 4.8 | Carboxylic acid (reference) |
| Phenol | 10.0 | Resonance-stabilized phenoxide |
| 2,2,2-Trichloroethanol | 12.2 | EWG stabilizes alkoxide |
| Water | 14.0 | Reference |
| 2-Chloroethanol | 14.3 | EWG, but farther from O |
| Methanol | 15.5 | Least substituted |
| Ethanol | 15.9 | — |
| Isopropyl alcohol | 16.5 | More EDG |
| tert-Butyl alcohol | 18.0 | Most EDG, most hindered |
| Cyclohexanol | 18.0 | Typical 2° alcohol |
Acidity order: Carboxylic acids > Phenols > Water > Alcohols
[!note] Within alcohols: methanol > 1° > 2° > 3° Both inductive and solvation effects push in the same direction for alcohols.
Basicity of Alcohols
In strong acid, alcohol acts as a weak base (lone pairs on oxygen accept proton):
$$\text{ROH} + \text{H}^+ \rightleftharpoons \text{ROH}_2^+$$
- Protonation is the first step in many alcohol reactions.
Acidity of Phenols
Phenols ($pK_a \approx 10$) are ~100 million times more acidic than typical alcohols ($pK_a \approx 18$).
Resonance Stabilization
The phenoxide ion is stabilized by delocalization of negative charge over oxygen and three ring carbons:
$$\text{C}_6\text{H}_5\text{OH} \rightleftharpoons \text{C}_6\text{H}_5\text{O}^- + \text{H}^+$$
Reactions with Bases
- Reacts with Na and NaOH (more acidic than water).
- Does NOT liberate $\text{CO}_2$ with $\text{Na}_2\text{CO}_3$ or $\text{NaHCO}_3$.
- Diagnostic: dissolves in NaOH but no effervescence with carbonate.
Effect of Ring Substituents
Substituents alter acidity by changing electron density on the phenolic oxygen via resonance and inductive effects.
Electron-Withdrawing Groups (EWG)
- Withdraw electron density from ring → weaken O electron density → stabilize phenoxide → increase acidity (lower $pK_a$).
- Ortho/para EWG most effective: resonance places +ve charge on carbon adjacent to $-\text{OH}$.
| Compound | $pK_a$ | Explanation |
|---|---|---|
| Phenol | 10.00 | Reference |
| 2-Nitrophenol | 7.20 | o-NO₂: resonance places +ve charge adjacent to O |
| 4-Nitrophenol | 7.20 | p-NO₂: same resonance effect |
| 3-Nitrophenol | 8.40 | m-NO₂: no resonance +ve charge adjacent to O; only −I |
Electron-Donating Groups (EDG)
- Donate electron density to ring → strengthen O electron density → destabilize phenoxide → decrease acidity (higher $pK_a$).
- Ortho/para EDG most effective: resonance places −ve charge on carbon adjacent to $-\text{OH}$.
| Compound | $pK_a$ | Explanation |
|---|---|---|
| Phenol | 10.00 | Reference |
| 4-Aminophenol | 10.30 | p-NH₂: resonance places −ve charge adjacent to O |
| 2-Aminophenol | 9.97 | o-NH₂: similar EDG effect; slight enhancement from intramolecular H-abstraction |
| 3-Aminophenol | 9.82 | m-NH₂: no resonance −ve charge adjacent to O; −NH₂ basicity aids proton removal |
[!tip] Drawing resonance structures To explain o-/m-/p- effects, draw resonance forms showing charge on the carbon adjacent to $-\text{OH}$. EWG creating +ve charge there increases acidity; EDG creating −ve charge there decreases acidity.
Preparation of Alcohols
1. Fermentation
- Yeast + sugars → ethanol + $\text{CO}_2$
- Yield: 12–15%; distillation increases to 40–50%.
2. Hydration of Alkene
- Alkene + dilute acid ($\text{H}_2\text{SO}_4$ or $\text{H}_3\text{PO}_4$).
- Markovnikov addition; dilute acid favors alcohol (Le Châtelier).
- Industrial: ethylene + steam, high T/P, catalyst ($\text{P}_2\text{O}_5$, etc.).
3. Nucleophilic Substitution of Haloalkane
- $\text{R-X} + \text{NaOH}/\text{KOH} \rightarrow \text{R-OH} + \text{X}^-$
- 1° → 1° alcohol (SN2); 2° → 2° alcohol (SN2/E2 compete); 3° → alkene (E2 dominates).
4. Grignard Reagent
- Formation: $\text{R-X} + \text{Mg} \xrightarrow{\text{dry ether}} \text{RMgX}$
- Formaldehyde → 1° alcohol; Aldehydes → 2° alcohol; Ketones → 3° alcohol.
- Anhydrous conditions essential — water destroys Grignard reagent.
5. Reduction of Carbonyl Compounds
- Covered in later topics (Sem 2).
Preparation of Phenols
Cumene Process (Industrial)
- Benzene + propene → cumene (alkylation)
- Cumene → cumene hydroperoxide (oxidation)
- Cumene hydroperoxide → phenol + acetone (rearrangement)
Laboratory Method
- Aromatic amine + $\text{HNO}_2$ → diazonium salt → phenol + $\text{N}_2$ (with $\text{H}_2\text{O}$).
Reactions of Alcohols
1. Reaction with Active Metals
$$2,\text{ROH} + 2,\text{Na} \rightarrow 2,\text{RONa} + \text{H}_2 \uparrow$$
- Reactivity: methanol > ethanol > 2° > 3°.
- 3° alcohols react very slowly (use K or NaH in THF).
2. Substitution to Haloalkane
| Reagent | Conditions | Product | Key Notes |
|---|---|---|---|
| HX | — | R-X | Phenol < 1° < 2° < 3° < benzyl; HCl < HBr < HI |
| HCl + ZnCl₂ | Lucas test | R-Cl | Differentiates 1°/2°/3° by cloudiness rate |
| PX₃ | — | R-X | Good yields 1°/2°; no rearrangement; poor for 3° |
| SOCl₂ | Pyridine | R-Cl | Gaseous by-products; retention of configuration |
Lucas Test: conc. HCl + $\text{ZnCl}_2$. $\text{Zn}^{2+}$ complexes with O lone pairs, weakens C–O bond. Cloudiness indicates alkyl chloride formation. 3° fastest, 1° slowest. Limited to alcohols <6 C.
3. Dehydration (Elimination)
- Reagents: conc. $\text{H}_2\text{SO}_4$ (180 °C), 85% $\text{H}_3\text{PO}_4$ (350 °C), $\text{Al}_2\text{O}_3$ (350 °C).
- Lower temp (~140 °C) gives symmetrical ethers.
- Follows Zaitsev's rule: most substituted alkene major product.
- Ease: 3° > 2° > 1° (carbocation stability).
- Usually E1 mechanism; rearrangements possible.
4. Oxidation
| Class | Reagent | Product |
|---|---|---|
| 1° | PCC, $\text{CH}_2\text{Cl}_2$ | Aldehyde |
| 1° | $\text{K}_2\text{Cr}_2\text{O}_7/\text{H}^+$ or $\text{KMnO}_4/\text{H}^+$ | Carboxylic acid |
| 2° | PCC, $\text{K}_2\text{Cr}_2\text{O}_7/\text{H}^+$, $\text{KMnO}_4/\text{H}^+$ | Ketone |
| 3° | — | Resistant (requires C–C cleavage) |
Chromic acid test: Orange → green/blue indicates 1° or 2° alcohol. No change with 3° alcohol, ketone, or alkane.
5. Esterification
- Alcohol + carboxylic acid $\xrightarrow{\text{H}^+}$ ester + $\text{H}_2\text{O}$ (equilibrium).
- Alcohol + acyl chloride → ester + HCl (irreversible, exothermic).
6. Iodoform Test
Positive for structures with $\text{-CH}_3$ bonded to $\text{-C=O}$ (or oxidizable to it):
- Ethanal, methyl ketones
- Ethanol (only 1° alcohol)
- 2° alcohols oxidizable to methyl ketones
Negative: methanol, 3° alcohols. Product: yellow $\text{CHI}_3$ precipitate.
Reactions of Phenols
O–H Bond Reactions
- Forms phenoxide with NaOH (more acidic than water).
- Phenoxide reacts with acyl chlorides/anhydrides to form esters.
- Phenol itself reacts poorly with carboxylic acids (weak nucleophile).
C–O Bond Limitations
- Phenols do NOT undergo acid-catalyzed elimination or SN2.
- Phenols are not easily oxidized (no H on carbinol carbon).
Electrophilic Aromatic Substitution
-$-\text{OH}$ is strongly activating and ortho-para directing.
- Reactions occur without Lewis acid catalyst.
Halogenation:
- Low T, non-polar: o- and p-monohalophenols.
- Aqueous, higher T: 2,4,6-trihalophenol (white precipitate for bromine).
Nitration:
- Dilute $\text{HNO}_3$, room temp: o- and p-nitrophenol.
- Conc. $\text{HNO}_3$: 2,4,6-trinitrophenol (picric acid).
Identification Tests
- Bromine water: rapid decolorization + white precipitate (2,4,6-tribromophenol).
- Aqueous $\text{FeCl}_3$: light purple complex (positive for phenols and enols).
Important Alcohols and Phenols
| Compound | Formula/SMILES | Uses |
|---|---|---|
| Methanol | CO |
Solvent, fuel, formaldehyde synthesis |
| Ethanol | CCO |
Beverages, solvent, fuel additive, antiseptic |
| Phenol | c1ccccc1O |
Disinfectant, plastics (Bakelite), nylon precursor |
| Propan-2-ol | CC(C)O |
Rubbing alcohol, topical antiseptic |
| Cyclohexanol | C1CCCCC1O |
Nylon-6,6 intermediate |
Industrial Applications
Ethanol
- Fermentation → 12–15% → distillation → 95% azeotrope → absolute (100%) with CaO.
- Fuel: Indianapolis 500 (since 2006), gasohol (10% in gasoline).
- Solvent, antiseptic, mouthwash.
Methanol
- Synthesis: CO + $\text{H}_2$ (high T/P).
- Solvent, fuel (Indy 500, 1965–2006), precursor to methyl compounds.
- Toxic: ~100 mL fatal.
Phenol → Nylon
- Hydrogenation: $\text{C}_6\text{H}_5\text{OH} + 3,\text{H}_2 \rightarrow (\text{CH}_2)_5\text{CHOH}$ ($\Delta G = -55.31\ \text{kJ/mol}$, 100 °C).
- Cyclohexanol/cyclohexanone → nylon-6 and nylon-6,6.
- Uses: textiles, airbags, carpet fibres.
Related Topics
- Carbonyl Compounds — Oxidation products of alcohols; reactants in Grignard synthesis
- Carboxylic Acids & Derivatives — Further oxidation products; esterification partners
- Amines & Amino Acids — Later topic with contrasting EDG/EWG effects and classification
- Stereochemistry — Optical isomerism of alcohols
Sources
- FAD1018 W7 — Alcohol and Phenol
- FAD1018 - Basic Chemistry II